Paint on UV'ed model with external editor
UV unwap the object, enter edit mode, go to the UV editor, click UVs at the bottom and finally click Export UV layout. It will give you an image with a UV template image, which you can paint over and then load back into blender.

Update: Based on your comment, try cutting the object in half using K and deleted one half. Then use the mirror modifier to restore the other side. Then position the viewport in the position you would like the final 2D texture coordinates to be. Now go into edit mode, press U and then unwrap from view. Follow the remainder of my steps from there.